1. Technical Field
The present invention relates to a gas sensor element for sensing the concentration of a specific component in a gas to be measured (to be simply referred to as a measurement gas hereinafter), to a method of manufacturing the gas sensor element, and to a gas sensor which includes the gas sensor element.
2. Description of Related Art
Conventionally, in the exhaust system of an internal combustion engine of a motor vehicle, there is generally arranged a gas sensor for sensing the concentration of a specific component (e.g., oxygen) in the exhaust gas from the engine. Further, based on the concentration of the specific component sensed by the gas sensor, various controls are performed which include, for example, an air/fuel ratio control and a temperature control of a catalyst used for treatment of the exhaust gas.
Moreover, the gas sensor may have a cup-shaped gas sensor element built therein. The gas sensor element includes: a solid electrolyte body that is formed of a solid electrolyte material (e.g., zirconia), which has oxygen ion conductivity, into a cup shape (or bottomed tubular shape); a measurement electrode provided on an outer surface of the solid electrolyte body so as to be exposed to the measurement gas (i.e., the exhaust gas from the engine); and a reference electrode provided on an inner surface of the solid electrolyte body so as to be exposed to a reference gas (e.g., air). Further, the gas sensor may be configured as an oxygen sensor to sense the electrical potential difference between the measurement and reference electrodes, which is caused by the difference in oxygen concentration between the measurement gas and the reference gas, and determine the concentration of oxygen in the measurement gas based on the sensed electric potential difference.
Furthermore, for ensuring prompt activation of the solid electrolyte body, the gas sensor may further have a heater built therein. The heater generates heat upon being supplied with electric power, thereby heating the gas sensor element. Further, it is possible to measure change in the complex impedance of the gas sensor element and control the temperature of the heater based on the measured change.
However, in the above case, in long-term use of the gas sensor, the measurement and reference electrodes will be repeatedly heated by the heater, causing cohesion of platinum particles that constitute the electrodes and thereby changing the grain-boundary capacitance. Consequently, the complex impedance of the gas sensor element will be increased, thereby lowering accuracy of the temperature control of the heater.
Japanese Patent Application Publication No. JP2003322631A discloses an oxygen sensor that includes a sensing element (or gas sensor element). In the sensing element, of the formation range of a reference electrode on an inner surface of the sensing element and the formation range of a measurement electrode on an outer surface of the sensing element, at least the formation range of the measurement electrode is set so as to be widest (or largest in circumferential length) at a high-temperature portion of the sensing element, where the temperature is highest in the sensing element, and to be narrowed (or reduced in circumferential length) as receding from the high-temperature portion.
More specifically, according to the disclosure of the above patent document, the formation range of the measurement electrode is set so as to be widest at a bottom portion of the sensing element and in the vicinity of the bottom portion. The bottom portion is most exposed to the exhaust gas (i.e., the measurement gas) and thus the temperature at the bottom portion is highest in the sensing element. Consequently, by setting the formation range of the measurement electrode so as to be widest at the bottom portion and in its vicinity, it is possible to secure the durability of the measurement electrode to heat. Moreover, by setting the formation range of the measurement electrode so as to be narrowed as getting away from the bottom portion, it is possible to suppress the influence of a non-activated part of the sensing element on the measurement electrode, thereby securing the responsiveness of the gas sensor.
However, the sensing element is heated by the exhaust gas over its entire circumference. Therefore, the temperature of the sensing element is constant in its circumferential direction.
Accordingly, the probability of the measurement electrode being broken at the high-temperature portion of the sensing element due to cohesion of the platinum particles is also constant in the circumferential direction. Therefore, even if the formation range of the measurement electrode is set as disclosed in the above patent document, it may be difficult to reliably improve the durability of the measurement electrode.
In addition, for the same reasons as the measurement electrode, it may also be difficult to reliably improve the durability of the reference electrode even if the formation range of the reference electrode is set as disclosed in the above patent document.
Moreover, in conventional gas sensor elements, the thickness (or film thickness) of the reference electrode is set to be constant. Consequently, upon exposure of a detection portion of the reference electrode to high temperature for a long period of time, cohesion of platinum particles may occur at the detection portion, thereby increasing the complex impedance of the gas sensor element. Accordingly, when the temperature of the heater is controlled based on the complex impedance of the gas sensor element, there may occur a deviation between a target temperature and the actual temperature of the heater. As a result, it may be impossible to stably and accurately sense the concentration of the specific component in the measurement gas.